ABSTRACT (Project 5: Maier, Neilson, Babst-Kostecka, Rasmussen) Soil contamination with trace elements in the vicinity of mining sites and smelters poses a serious threat to humans and the environment around the globe. Revegetation of mine tailings to minimize the dispersal of pollutants via wind or ground water (i.e. phytoremediation) is a promising ?green? and low-cost intervention to toxic exposures. Unfortunately, plant growth is inhibited on most tailings, making it necessary to provide additional healthy substrate for seed germination and seedling growth. The current practice is the installation of an uncontaminated soil-gravel-rock cap over the tailings prior to plant seeding, with stockpiles of such material being readily available, a technology known as ?cap and plant?. However, critical knowledge gaps regarding plant-soil interactions have prevented a broad and efficient implementation of this technology. The primary objective of this project is to identify the optimal strategy for generating a lasting vegetation cover at hazardous mining sites. The guiding hypothesis is that the biophysicochemical properties of capping material are critical for the development of robust root systems that can propagate into the underlying contaminated mine tailings. Using an innovative experimental design, this project will develop specific soil health indices and assess the effects of capping material depth and quality on root system architecture in a prospective plant species known as saltbush (Atriplex lentiformis) for phytoremediation. Various capping materials from stockpiled overburden and adjacent natural deposits will be tested in three consecutive greenhouse studies that bring together advanced ecological, genomic, and soil health assessments. Specifically, root system development will be monitored using a noninvasive phenotyping method based on rhizotrons, filled with different combinations of capping material and mine tailings from Superfund sites across the US Southwest. Once the optimal soil and plant parameters are identified, possibilities to amend existing but low-quality capping material in a cost-effective way will be explored. This project will yield an unprecedented mechanistic understanding of concurrent changes in plant root system architecture and function in response to the quality and depth of capping materials used for mine tailing restoration. Capitalizing on this knowledge, specific guidelines towards the remediation of hazardous sites will be developed and directly transferred to the mining industry and regulators. As such, the project outcome will be valuable for the economy and society. In addition, our findings will serve as a global template for mitigating human and environmental health issues in areas affected by mines and smelters.